You can see the voice-calling option by tapping on a friend’s name, then hitting the button at the top-right of the Contact Info screen. If they’re online, the “Free Call” button can be used to dial them directly, no phone number required. Otherwise this button is grayed out.

The company commonly uses its standalone apps with their smaller user bases as a testing ground for new features before they make their way into Facebook’s main application.

With voice calling, it’s notable that Facebook has not been building on top of its existing Skype partnership, which in the past had powered a voice calling test on Facebook’s desktop site.

More importantly, the move is pitting Facebook against the phone’s default calling application, as Josh Constine pointed out last month. Because people’s Facebook networks tend to represent their real-world friendships and connections – meaning those people they’re likely to jump on the phone with – Facebook is in a position to actually have an impact in terms of reducing the number of voice minutes a person needs to have on their calling plan.

It would also provide Facebook with another piece of data about its users, by informing the network who a user’s “real” friends are. That can help it refine its relevancy algorithms for everything from the News Feed to ads and even to Facebook’s newly launched Graph Search.